In critically ill and unstable patients assessment of oxygen delivery to the tissues is of vital importance. If oxygen delivery is inadequate, early interventions to optimise oxygen delivery may prevent multiple organ failure and death1. These interventions include administration of intravenous fluids, inotropes (that stimulate heart contraction) and support of ventilation to improve oxygenation of blood.
Central venous or mixed venous blood oxygen saturations reflect the adequacy of oxygen delivery to the parts of the body from which the blood has drained. Mixed venous blood (blood in the right ventricle and central and peripheral parts of the pulmonary arteries) offers the best assessment of the adequacy of oxygen delivery to the whole body. However, central venous blood (blood in the internal jugular, subclavian, femoral and brachiocephalic veins, the inferior and superior vena cava and the right atrium) can be used as a surrogate of the adequacy of oxygen delivery to the whole body.2 
Conventionally, assessment of oxygen delivery by venous saturation measurement is generally undertaken by placing a catheter in a central vein or pulmonary artery from which blood is withdrawn. Oxygen saturation of the withdrawn blood is then measured by a blood gas machine. Alternatively, a fibre-optic catheter can be placed in the central vein or pulmonary artery and the oxygen saturation can then be directly measured by optical methods. An approach such as this involving the insertion of an intravenous fibre-optic catheter and direct measurement of oxygen saturation by oximetry is discussed in U.S. Pat. No. 5,673,694 to Rivers.
Both of these approaches involve significant limitations as they require a skilled doctor to insert the catheter, they involve the expense of the blood gas machine or fibre-optic catheter, there is significant risk of adverse events associated with catheter insertion (pneumothorax, infection, bleeding, arrhythmia and tamponade) and finally, there is a delay in obtaining the venous blood saturation while the catheter is inserted.
In earlier published International Patent Publication No. WO2008/134813 (the disclosures of which are included herein in their entirety by way of reference) the present inventor described a non-invasive method to directly measure blood oxygen saturation (such as central venous and mixed venous blood oxygen saturation) by placing a light oximeter device on the skin over deep vascular structures. Pulse oximetry, using red and infrared light sources, is an established technique to measure haemoglobin oxygen saturation of blood vessels in the skin. Deoxyhaemoglobin (Hb) absorbs more of the red band while oxyhaemoglobin absorbs more of the infra-red band. In the earlier International Patent Publication preferred wavelengths of red light of from about 620 nm to about 750 nm and of infra-red light of from about 750 nm to about 1000 nm were disclosed. In pulse oximetry light is first transmitted through the tissues and the intensity of the transmitted or reflected light is then measured by the photo-detector. The pulse oximiter determines the AC (pulsatile) component of the absorbance at each wavelength and the amount of the red and infrared AC components is determined, which is indicative of the concentration of oxyhaemoglobin and deoxyhaemoglobin molecules in the blood. The ratio of oxygenated haemoglobin to total haemoglobin indicates the oxygen saturation of the blood.
In WO2008/134813 the present inventor demonstrated that by utilising the pulsatile nature of the deep vascular structures to generate a plethysmographic trace it is possible to accurately locate the emitter and receiver elements to optimise the signal detected and to thereby do away with the need for concurrent ultrasonography and measurements from more than one location. The individuality of the plethysmography in the technique described was used to identify that the signal is arising from the vascular structure of interest and to filter out signals arising from other interfering chromophores, such as small blood vessels and surrounding tissues.
The present inventors have now determined that improvements in accuracy and reliability of blood oxygen saturation determination by oximetry from deep vascular structures can be made by adopting one or more of (a) selecting optimal wavelengths for determination of light absorption by haemoglobin in the blood (for example from about 1045 nm to about 1055 nm and from about 1085 nm to about 1095 nm); (b) locating the oximetry emitter and receiver elements within the external auditory canal of the patient; (c) increasing distance between emitter and receiver elements up to a threshold level; and (d) angling the emitter element at an angle of approximately 45° relative to the angle of the receiver element.
Although there is disclosure in Roggan et al3 that when testing the optical properties of blood in the wavelength range 400 to 2500 nm the scattering coefficient decreased for wavelengths above 500 nm, there is no disclosure or suggestion provided that improved accuracy and reliability may be obtained in conducting oximetry of blood in deep vascular structures at wavelengths of from about 1045 nm to about 1055 nm and from about 1085 nm to about 1095 nm, to thereby determine the level of oxygen saturation.
In a study in relation to the use of medical lasers for tissue ablation that assessed the relationship between wavelength and skin penetration depth (across the range of wavelengths 400 to 2000 nm) it was found that maximal skin penetration was achieved at the wavelength of 1090 nm. Similar results were demonstrated for other biological tissues including bone, brain and liver.4-6 There is, however, no disclosure or indication provided in these papers that improved accuracy and reliability of blood oximetry conducted to determine the level of oxygen saturation in deep vascular structures can be obtained by utilising wavelengths of from about 1045 nm to about 1055 nm and from about 1085 nm to about 1095 nm.
Further, although U.S. Pat. No. 5,213,099 to Tripp discloses a device intended to be inserted in the ear canal for monitoring the physiological condition of pilots and other air crew flying in high performance aircraft the disclosure relates to a device intended to monitor blood oxygen saturation within blood vessels at the surface of body cavities such as the ear canal. However, Tripp makes no suggestion that by inserting an oximetry device into the ear canal it may be possible to monitor blood oxygen saturation within a deep vascular structure.
The present invention may overcome or at least to some extent ameliorate problems associated with prior art methods of determining oxygen saturation in deep vascular structures. Other desirable objectives of the present invention will become apparent from the following detailed description thereof.